#include<iostream>

using namespace std;

class Sum{
  public:
    Sum(){
      _ret+=_i;
      _i++;
    }
  static int ReturnRet()
  {
    return _ret;
  }
  private:
  static int _ret;
  static int _i;
};
int Sum::_ret = 0;
int Sum::_i = 1;
int main()
{
   int n = 0;
   cin>>n;
   Sum a[n];
   cout<<Sum::ReturnRet()<<endl;
   return 0;
}
